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I would suggest that either the battery has boiled or the battery has a crack and it has allowed the sulphuric acid to leak (rotten egg smell)
If it has leaked I would be getting the battery out immediately (stop acid attack on metal) and neutralising the area with a solution of Bicarbonate of Soda and water - if there is battery acid around the sod bicarb solution with fizz/bubble vigorously.
Continue to apply sod bicarb solution until all bubbling ceases and then thoroughly wash down all areas that the sodbicarb/battery acid might have got to.